In integrated circuits processes, the LOCOS (local oxidation of silicon) technique has been widely and mainly used for isolation between devices. This traditional technology normally causes a serious bird's beak effect and results in a larger required area for circuit components. Therefore, how to reduce the bird's beak effect is one of the main tasks in nowadays integrated circuits technique.
The conventional LOCOS technique includes the following steps:
1) pushing a wafer, which is first cleaned, into a furnace;
2) causing the furnace in an oxygen atmosphere at a temperature from about 900.degree. C. to about 1100.degree. C. to form a buffer oxide layer from about 100 .ANG. to about 300 .ANG.;
3) growing on the buffer oxide layer a silicon nitride layer having a thickness from about 1000 .ANG. to about 2000 .ANG. under a temperature from about 650.degree. C. to about 850.degree. C. by a low-pressure chemical vapor deposition;
4) applying a photoresist on the wafer;
5) applying a mask to the wafer;
6) etching undesired parts of the silicon nitride layer by a dry etch technique according to the mask;
7) forming the field oxides on where the undesired parts of the silicon nitride layer are etched by the wet oxidation technique at about 950.degree. C. to about 1100.degree. C. for about 1 hour to about 3 hours.
This conventional method has an advantage of being a simple procedure. Whereas, with the trend of the smaller and smaller size of the circuit component, this conventional method cannot meet our demand when the size of the circuit component is smaller than 0.5 .mu.m.
For example, the buffer oxide layer obtained by exposing the wafer to O.sub.2 has a thickness ranging from about 315 .ANG. to about 415 .ANG.. After the Si.sub.3 N.sub.4 layer (about 1050 .ANG. to about 1300 .ANG.) is grown on the buffer oxide layer and then the field oxides (about 6000 .ANG. to about 7000 .ANG.) are grown in the wet oxygen atmosphere, as shown in FIG. 1, it is found that each side of the bird's beak according to this conventional LOCOS technique is about 5000 .ANG.. That is to say, the prearranged distance should be about 1.25 .mu.m to about 1.5 .mu.m so that the required area is enough.
For a further explanation, according to the conventional LOCOS technique, one wafer is first cleaned (having a diameter about 6 inches, a direction (100), and a resistance about 15 .OMEGA.-cm to about 25 .OMEGA.-cm; and being p-type), grow a buffer oxide layer about 100 .ANG. on the wafer at about 900.degree. C. at a 10% O.sub.2 and 90% N.sub.2 atmosphere in a furnace, and then a Si.sub.3 N.sub.4 layer about 1200 .ANG. is grown on the wafer. After the undesired parts of the Si.sub.3 N.sub.4 layer are removed, the wafer is pushed into a wet oxygen atmosphere at about 980.degree. C. for 80 minutes to form thereon the field oxides and then the final product is obtained.
FIG. 2 is the SEM photograph of the above-mentioned example according to the conventional LOCOS technique, and it is found that by the conventional LOCOS technique, the length of the bird's beak (i.e., X.sub.1) is 541.7 nm (5417 .ANG.). This is not so satisfactory.
For reducing the bird's beak effect, there are many researches centered on this subject, such as the Semi-Recessed Oxide LOCOS, the Fully-Recessed Oxide LOCOS, and the Poly Buffered LOCOS. These methods can more or less reduce the bird's beak effect, but they have a common disadvantage: these methods need many extra steps which increase costs. Besides, these steps are also relatively complicated and troublesome.
It is therefore attempted by the Applicant to deal with the above situation encountered by the prior art.